The present invention relates to a plasma etching apparatus used in micropattering in fabrication of semiconductor devices.
In micropatterning in fabrication of semiconductor devices, plasma etching using a high-frequency glow discharge of a reactive gas is mainly used. In particular, in the process requiring high-precision control of dimensions and shapes, reactive ion etching (RIE) is mainly used wherein a sample (semiconductor wafer) is placed on a cathode electrode applied with high-frequency power and directional etching is performed with accelerated ions in a direction perpendicular to an electrode surface upon biasing of the cathode electrode to a negative voltage.
RIE etching characteristics change depending on types and amounts of radicals produced in a gaseous phase and the energy and density of ions incident on the cathode electrode which supports an object to be etched. An etching rate in normal conditions is a maximum of about several tens of nanometers/min. When high-frequency power is increased to increase the etching rate, the amount of radicals and an ion current density are increased, and ion energy is also increased, thus degrading selectivity with respect to an etching mask and an underlying layer and hence damaging semiconductor crystals. In order to increase the throughput, a large apparatus capable of processing a large number of semiconductor wafers is required. RIE poses a problem of etching shape errors caused by disturbing ions produced upon collision between accelerated ions and the neutral gas in an ion sheath on the cathode electrode when micropatterning is further advanced.
In order to solve the above problems, a so-called magnetron etching apparatus is proposed to increase the throughput by applying a magnetic field combined with a high-frequency electric field, thereby generating a plasma. An example of the magnetron etching apparatus is described in U.S. Pat. No. 4,422,896 issued to Walter H. Class et al. A similar magnetron etching apparatus is also described in Japanese Patent Laid-Open (Kokai) No. 57-159026 by Haruo Okano et al. In these apparatuses, electrons are drifted in a direction perpendicular to the electric and magnetic fields upon application of a magnetic field in a direction perpendicular to the high-frequency electric field. As a result, collision of the electrons with the gas is activated, and a discharge plasma density is increased. Since the density of an ion current supplied to the sample on the cathode electrode is increased, the etching rate can be increased to about ten times as compared with the conventional RIE apparatus. Therefore, the magnetron etching apparatus can have a sufficiently high throughput even if one substrate is etched, thereby obtaining a compact apparatus.
Another conventional etching apparatus using a coil as a magnetic field applying means is also known. An example of this etching apparatus is described in a Paper "SiO.sub.2 High-Speed Etching", Kyungshik Kim et al., 7th Dry Process Symposium, p. 95, 1985. A magnetron etching apparatus using a coil is also described in Japanese Patent Laid-Open (Kokai) No. 63-17530 by Owen Wilkinson.
As described in Japanese Patent Publication No. 12346 and a paper "Double-source Excited Reactive Ion Etching and Its Application to Submicron Trench Etching", Masaaki Sato and Yoshinobu Arita, Extended Abstracts of the 18th (1986 International) Conference on Solid State Devices and Materials, Tokyo 233 (1986), which describe triode plasma etching apparatuses for independently controlling the ion energy, the current density, and the radial concentrations, an additional cathode is arranged to oppose a cathode electrode on which an object to be etched is placed, and a grid comprising a mesh or perforated plate is arranged as a common anode electrode between the cathode electrodes.
FIG. 7 is a schematic view for explaining the triode plasma etching apparatus. Reference numeral 21 denotes a vacuum chamber; 9, a vacuum pump connected to the vacuum chamber 21; 8, a gas supply system connected to the vacuum chamber 21; 1 and 2, first and second cathode electrodes arranged in the vacuum chamber 21; 6 and 7, high-frequency power sources respectively connected to the cathode electrodes 1 and 2; 5, blocking capacitors arranged between the high-frequency power source 6 and the cathode electrode 1 and between the high-frequency power source 7 and the cathode electrode 2, respectively; 3, a grid arranged between the cathode electrodes 1 and 2; and 4, an object to be etched, which is placed on the cathode electrode 1. In the triode plasma etching apparatus, a discharge area opposite to the cathode electrode 2 is divided by the grid 3 from a discharge area opposite to the cathode electrode 1 on which the object to be etched is placed. Upon application of high-frequency power to the cathode electrode 2, decomposition and ionization of the gas are accelerated to increase the density of active radicals and a plasma density of the discharge area of the cathode electrode 1 which supports the object through the grid 3. Therefore, the amount of ion current supplied to the object and the amount of active radicals can be increased. As compared with the counterelectrode type plasma etching apparatus, the etching rate can be increased to 2 to 4 times. In addition, the width of the ion sheath formed on the surface of the object to be etched is decreased with an increase in ion current density. Therefore, disturbance of the ions caused by collision with gas molecules in the ion sheath can be suppressed. Therefore, etching in a depth about 10 times the width of 0.25 .mu.m of a submicron area can be performed with high precision.
In the conventional magnetron RIE apparatus using a permanent magnet, the magnetic field generated by the permanent magnet is fixed, and flexibility of etching conditions is degraded. For example, the ion energy and its current density cannot be independently controlled. In addition, high-precision etching uniformity control cannot be performed. In the apparatus using the coil, although flexibility of the etching conditions can be improved as compared with the apparatus using a permanent magnet, it is difficult to independently control the ion energy and its current density.
In various types of magnetron etching apparatuses in which magnetic fields are changed as a function of time, e.g., a coil or a permanent magnet is physically moved, or a current supplied to a coil is changed as a function of time, a change in magnetic field is slower than a change in high-frequency electric field as a function of time. This causes variations in ion energy and its directivity, thus causing element damage and degradation in etching shapes.
In the conventional triode etching apparatus, flexibility of the etching conditions is increased, and the ion energy and its current density can be independently controlled. In addition, discharge variations as a function of time can be eliminated, so that damage and degradation of etching shapes can be eliminated. However, the throughput of the triode etching apparatus is not sufficient as compared with the magnetron etching apparatus due to the following reason. The plasma generated in the discharge area of the cathode electrode 2 is diffused to the discharge area of the cathode electrode 1, and a large amount of the plasma generated in the discharge area of the cathode electrode 2 is not guided to the object 4 but is diffused to be recombined. Therefore, even if the high-frequency power applied to the cathode electrode 2 is increased, an increase in density of the ion current supplied to the object 4 is saturated. In addition, nonuniform etching also occurs by the influence of the grid 3.